An achromatic prism is made by combining two prisms ${{P}_{1}}({{\mu }_{v}}=1.523,\,{{\mu }_{r}}=1.515)$ and ${{P}_{2}}({{\mu }_{v}}=1.666,\,{{\mu }_{r}}=1.650)$; where $\mu $ represents the refractive index. If the angle of the prism ${{P}_{1}}$ is 10°, then the angle of the prism $P_2$ will be